1. Field of the Invention
This invention refers to a colorless soda-lime-silica glass composition, and more particularly relates to a colorless glass composition that utilizes less than 0.03 wt % of ferric oxide and 0.05 to 1 wt % of titanium oxide to produce a colorless glass with a high visible light transmission for use in the construction, appliance, glazing and automotive industries.
2. Description of the Related Art
The well-known formulations of lime glass, which comprise a mixture of sodium oxide (Na2O), lime (CaO) and silica (SiO2), known also as “soda-lime-silica glass”, are used in the production of “flat glass” for architectural use and, for use in the automotive industry.
For automotive use it is highly desirable that the glass have a high level or percentage of visible light transmission, in order to provide the driver with good visibility of his surroundings, thus complying with the norms of automotive safety.
Several patents have been developed for obtaining colorless glass, using a standard glass composition. For construction as well, it is highly desirable that the glass has a high level or percentage of visible light transmission, in order to provide a good vision area.
It is also desirable that the glass has the necessary absorption properties to absorb damaging infrared (IR) and ultraviolet (UV) solar radiation, so as to reduce the excessive heating of the houses or vehicles on sunny days.
Generally, a glass composition contains ferrous and ferric oxides. The balance between ferrous and ferric oxide has a direct effect on the color and transmittance properties of the glass. The iron is generally present in the glass as both ferrous oxide (FeO) and ferric oxide (Fe2O3) imparting to the glass a clear green-blue color.
In this manner, in a glass composition, the total amount of iron is present as both ferric oxide (Fe2O3) and as ferrous oxide (FeO), since even when pure ferric oxide is used in the basic raw material during the glass forming process, a portion of the ferric oxide is reduced and is transformed into ferrous oxide.
Normally, the total amount of iron in the glass is expressed as ferric oxide (Fe2O3) independent of the reduction state of iron. It is also standard in this industry to express the quantity of ferrous or ferric oxide as a percentage of the total iron, namely:
            %      ⁢                          ⁢              Fe                  +          2                    ⁢                          ⁢              (        FERROUS        )              =                  Fe0        ×        100                    Total        ⁢                                  ⁢                  Fe          2                ⁢                  O          3                                %      ⁢                          ⁢              Fe                  +          3                    ⁢                          ⁢              (        FERRIC        )              =                            Fe          2                ⁢                  O          3                ×        100                    Total        ⁢                                  ⁢                  Fe          2                ⁢                  O          3                    
The iron oxides (ferric and ferrous) impart different optical properties to the glass, the total quantity of iron present and its equilibrium as ferric or ferrous have a direct impact on the color, light transmission and absorption of infrared and ultraviolet radiations.
The ferric oxide absorbs ultraviolet energy (low transmission level), having at the same time high visible light and infrared energy transmissions.
By contrast, ferrous oxide absorbs infrared energy (low transmission), with a high ultra-violet transmission, and a lower level of visible light transmission and possesses a more intense blue color.
Therefore, the greater the quantity of Fe2O3 present in the glass, the greater will be the absorption of ultraviolet radiation, and the light transmission is also increased but, as the content of FeO is increased as a result of the chemical reduction of Fe2O3, the absorption of the infrared radiation will increase, while the absorption of the ultraviolet radiation is decreased and the light transmission is also undesirably decreased.
The greater the concentration of FeO in relation to Fe2O3, results in a change in the color of the glass. The shift toward a higher concentration of FeO in relation to the Fe2O3 causes a change of color of the glass from a yellow to yellow-green to a darker blue-green which is undesirable, since it reduces the light transmission of the glass.
Therefore, to manufacture a glass with predetermined properties and color, one must have the correct amount of total iron and correct proportion of Fe2O3 and FeO, taking into account that what is increased on the ferrous side, will diminish on the ferric side, and consequently one must arrive at a compromise of properties since improving (lowering) the value of one property will worsen (raise) the value of the other properties.
In order to increase the absorption of the infrared and ultraviolet radiation without sacrificing the transmission of the visible spectrum, it has been necessary to lower the total content of the iron which is highly reduced from ferric to ferrous, to less than 0.70% of total iron expressed as Fe2O3.
Depending on the state of reduction of the glass, the coloring changes are as follows:
Low Ferrous (12%)—Yellow—High Light Transmission (High Ferric)
Yellow-Green
Green-Yellow
Green (Desirable)
Green-Blue
Blue-Green
Blue
High Ferrous (75%)—Amber—Low Light Transmission (Low Ferric)
Additionally, it is known that the oxides of titanium, molybdenum and cerium, principally of cerium, are also colorants, and when they are used in combination with the Fe2O3, it is possible to obtain an additional reduction of the ultraviolet light transmission to a point where the sought for visibility transmission is achieved. It does, however, suffer from the disadvantage of its high cost, which makes the formulation very expensive, and has a tendency to oxidize the iron to Fe2O3.
On the other hand, the effects produced by the use of titanium dioxide include the fact that TiO2 greatly increases the refractive index, increases the absorption of light in the ultraviolet region, and lowers the viscosity and surface tension. From the data on the use of titanium dioxide in enamels, it has been noted that TiO2 increases the chemical durability and acts as a flux. In general, clear glasses containing titanium dioxide may be found in all of the common glass-forming systems, such as borates, silicates, and phosphates. The various regions of glass formation for systems containing titanium dioxide are not grouped in any one place, since the organization of the discussion is based more on the properties in uses of glasses containing titanium dioxide than on their constitution alone.
Many patents have been issued on colored glass compositions with infrared and ultraviolet radiation absorbing characteristics. U.S. Pat. No. 5,030,593 issued to Robert Heithoff on Jul. 9, 1991, discloses a clear glass, with the substantial absence of color in transmittance, and with an attractive, bright, edge coloration compatible with wood tone surroundings is achieved in a glass having greater than 85 percent, preferably greater than 87 percent, luminous transmittance by minimizing the amount of iron oxide present in the glass and including very small amounts of selenium and optionally cobalt oxide. Various warm, wood tone compatible colors at the edge can be attained, including a “honey” color and almost neutral grays.
U.S. Pat. No. 5,030,594 issued to Robert Heithoff on Jul. 9, 1991, is related to a clear glass, with the substantial absence of color in transmittance, and with an attractive, bright, pure azure edge coloration is achieved in a glass having greater than 87 percent, preferably greater than 90 percent, luminous transmittance by using very small amounts of iron oxide as the sole essential colorant, with the ratio of iron in the ferrous state to total iron being at least 0.4.
U.S. Pat. No. 5,346,768 issued to Ernst Winter et al, on Sep. 13, 1994, discloses a soda-lime glass containing vanadium, in particular a flat glass produced by the float glass process with high UV absorption for wavelengths below 350 nm. The purpose of the invention is to provide a simple and cost effective producible flat glass, in particular a glass produced by the float glass process with high UV absorption without reduction of the neutrality of color or significant reduction of the total light transmittance (as defined in DIN 1249, Part 10).
U.S. Pat. No. 5,656,559 issued to Jean-Marie Combes et al on Aug. 17, 1997, is related a soda-lime-silica glass composition which enables panes to be produced that have a lower coloration than that of the known glasses and a better absorption of the infrared radiation, even with the same iron content in the form of FeO and the same thickness.
The U.S. Pat. No. 6,218,323 issued Joachin Bretschneider and Hubert Drexler on Apr. 17, 2001, describes a neutral-colored soda-lime-silicate glass with high light transmission in the visible region. The glass has a basic composition which contains at least the following constituents: SiO2, 66-75 weight %; Na2O, 10-20 weight %; CaO, 5-15 weight %; MgO, 0-6 weight %; Al2O3, 0-5 weight %; and K2O, 0-5 weight %; and incorporates a colorant portion comprising the following constituents: Co, 0.1-1 ppm; Fe2O3, 0.1 to req. 0.03 weight % (total iron content); and FeO/Fe2O3, <0.4. The glass possesses a light transmittance (illuminant D 65 according to DIN 67 507) of at least 89% at a reference thickness of 4 mm.
Finally, the U.S. Pat. No. 6,258,740 issued to Hiroshi Machishita, et al, on Jul. 10, 2001, describes an ultraviolet ray absorbing soda-lime glass. This glass contains less than 0.10 wt % of iron in terms of Fe2O3, which is optionally contained as an impurity in the glass, 0.7-2.6 wt % of CeO2, 0-1.3 wt % of TiO2, 0-0.12 wt % of V2O5, 0.08-0.30 wt % of sulfur in terms of SO3, and 0-0.0025 wt % of CoO. The glass at a thickness of 5 mm is not higher than 10% in ultraviolet radiation transmittance, is not lower than 80% in visible light transmittance, and is from 530 to 575 nm in dominant wavelength. The glass is transparent and clear, superior in ultraviolet ray absorption capability, and high in visible light transmittance.
With respect to the foregoing, many others patents and papers have been published on colored glass compositions with infrared and ultraviolet radiation absorbing characteristics, to describe the importance of the equilibrium between ferrous and ferric oxides in glasses. For example “N. E. Densem; The equilibrium between ferrous and ferric oxides in glasses; Journal of the Society of Glass Technology, Glasgow, England, May 1937, pp. 374-389”; “J. C. Hostetter and H. S. Roberts, “Note on the dissociation of Ferric Oxide dissolved in glass and its relation to the color of iron-bearing glasses; Journal of the American Ceramic Society, USA, September, 1921, pp. 927-938”.
However, as it can be clearly appreciated from the above patents, in order to express the visible light transmission characteristics of a glass, it is necessary to take into account the following three main points:
1. The thickness at which it is measured, since the transmission of UV, visible light and infrared decrease in direct relation with the increase of the thickness of the glass.
2. The wavelengths of the different zones, for example the UV transmission is considered to be from 300 to 400 nm (General Motors); from 300 to the half the value of 400 nm (Ford), since the other half is transferred to the visible light; from 300 to 390 nm (PPG's U.S. Pat. No. 5,240,866); from 280 to 380 nm in IS09050; as well as if the increments were from 2.5, 5 or 10 nm. Consequently, there will be different values when measuring the ultraviolet transmission for the same product.
3. The Standard utilized in respect to the solar energy, should be established beforehand, for example: “CIE PUBL:” 40; and the air mass, Perry & Moon Air Mass=2, Air Mass=1.0 or air mass 1.5 as recent GMW3136 standard.
According to the present invention, there is provided a soda-lime-silica glass composition that utilizes ferric oxide and titanium oxide to produce a glass suitable for use in the appliances, glazing, construction and automotive industry with a thickness of about 3 to 4 mm and preferably about 3.2 mm.
Papers have been written on the behavior of the Titanium TiO2 in the transmission of colorless glasses. i.e. Striple, J. H. “Titanium dioxide its effect on the transmission of various glasses”, The glass industry/April 1964, pp 193-196. The author comments that the TiO2 optically changes the light transmission to a major wavelength, extending the transmission range in the infrared and absorbing more in the ultraviolet.
The more stable form of titanium in the glass is the tetravalent (Ti4+) which is colorless with only the trivalent form (Ti3+) producing color. However, the color produced by Ti3+ has not been found in soda-lime-silica glasses.
M. D. Beals in the paper, “Effects of Titanium Dioxide in Glass”, “The glass industry, September, 1963, pp 495-53, describes the interest that has been shown the titanium dioxide as a constituent of glasses. The effects produced by the use of titanium dioxide included the comments that TiO2 greatly increases the refractive index, increases the absorption of light in the ultraviolet region, and that is lowers the viscosity and surface tension. From the data on the use of titanium dioxide in enamels, they noted that TiO2 increases the chemical durability and acts as a flux. In general, clear glasses containing titanium dioxide may be found in all of the common glass-forming systems (borates, silicates, and phosphates). The various regions of glass formation for systems containing titanium dioxide are not grouped in any one place, since the organization of the discussion is based more on the properties than use of glasses containing titanium dioxide than on their constitution alone.
From the above, the present invention is related to a colorless soda-lime-silica glass composition which contains less than 0.03 wt % of Fe2O3 and from 0.05 to 1 wt % of TiO2. The glass at 3.2 mm is higher than 87% in visible light transmittance (Ilum D65 according ISO 9050) and higher than 89% more preferable.